This invention relates to methods and apparatus for electrostatically charging a surface and more particularly to improved corona generating methods and apparatus therefore usable in electrostatic recording and reproducing equipment or in any other application where it is desirable to efficiently charge a selective medium in a controlled manner.
In electrostatic recording and reproducing processes, such as the electrophotographic process known as xerography, it is necessary to sensitize a photoreceptor structure by charging at least one surface thereof to a potential which is preferably uniform. Subsequent to or simultaneously with the sensitizing of the photoreceptor structure in such electrophotographic processes, the photoreceptor structure is exposed so that a photosensitive layer therein is rendered selectively condutive whereupon a latent electrostatic image is formed which may be then developed using conventional electrophotographic techniques. The developed image is then transferred onto a copy sheet on which it is rendered permanent by means of a fixing process.
In the above-noted electrophotographic process, electrostatic charging techniques are generally relied on to accomplish such necessary processing steps as laying down of an initial charge on an imaging surface, the transfer of an electrostatically toner image from a reusable photoreceptor structure to a transfer member, the tacking and stripping operations associated with such transfer member, and various other conditioning processes whereby charges are improved or modified on a xerographic medium.
While many forms of acceptable techniques for electrostatically charging a surface are known, corona discharge techniques have generally been preferred in applications such as those mentioned above because such techniques are particularly well suited to applying an electrostatic charge to a moving surface and the use of corona discharge techniques allows for a selected surface to be rapidly charged to a relatively high potential. Furthermore, since corona generating apparatus generally employ a wire like electrode, they are advantageous because the charging process acts to impose a potential level on the surface being charged which tends to be more uniform than that obtained from other surface charging techniques. Conventional forms of corona generating apparatus are illustrated in U.S. Pat. Nos. 2,836,725 and 2,879,395 and generally comprise one or more wire like electrodes, known as coronodes, horizontally disposed above the surface to be charged and a shield which is usually a conductive member which may take a plurality of different structural forms, partially disposed about the coronode. In one conventional mode of operation, a high voltage D.C. power supply is connected to the coronode with the requisite polarity for the charging operation which is desired, while a conductive layer associated with the surface to be charged is grounded, as are the other terminal of the power supply and the shield.
Another form of energization for corona discharge devices of the above-noted type is disclosed in U.S. Pat. Nos. 2,777,957 and 2,879,395 wherein an elongated coronode is coupled to an alternating current source, while concurrently grounding the substrate on which the surface to be charged rests. Control of the charge deposited by such an arrangement is achieved by using a so-called scorotron grid biased to a D.C. potential and located between the coronode and the surface to be charged. Alternately, the shield may be held at a specific potential above ground while applying an A.C. signal to the coronode.
In any of the above-noted arrangements, it is well known to those skilled in the art that the magnitude of the charging current delivered at any given moment to the photoconductive surface is effected by many factors among which are the potential of the surface below the corona discharge device and ambient conditions of relative humidity, temperature, etc.
Arrangements for achieving dynamic control of the rate at which charge is deposited by a corona discharge device onto a photoconductive surface are desirable. Several prior art methods of accomplishing this are known.
In U.S. Pat. No. 3,062,956, the scorotron grid or screen, which is interposed between the coronode and the imaging surface, is held at a D.C. bias potential which is varied in response to charging current to maintain the charging current constant.
In U.S. Pat. No. 3,335,274 there is disclosed a feedback arrangement in which the charging current to the imaging surface is monitored and made to control the high voltage potential applied to the coronodes, to thereby hold the charging current constant. This arrangement has the disadvantage of requiring the control of relatively high voltages which are used to energize the coronodes.
U.S. Pat. No. 3,335,275 is of interest in that it shows an arrangement for terminating the charging action of a corona device when the potential of the surface to be charged reaches a preselected value.
It has also been known that a D.C. bias may be superimposed on a corona discharge device energized by applying an A.C. signal to its coronodes, as shown in U.S. Pat. No. 3,275,837. In connection with such biasing arrangements, it is further known to those skilled in the art that the charging current delivered to an imaging surface from a D.C. biased A.C. energized corona device may be varied by changing the value of this D.C. bias.
Copending commonly assigned application, Ser. No. 491,895, discloses an arrangement whereby the D.C. bias on an A.C. pretransfer corona device may be changed in response to manual selection by a xerographic machine operator in order to improve copy quality.
Copending application Ser. No. 531,131 also commonly assigned, discloses a control arrangement for a corona device of a xerographic machine wherein the D.C. biasing voltage applied to the corona generator shield is varied in response to a signal detected by a voltage response electrometer probe.